An electrophotographic image forming apparatus which prints an image by electrophotography has a drum-shaped electrophotographic photosensitive body (to be referred to as a photosensitive drum hereinafter), as is known. In forming an image, the surface of the photosensitive drum is uniformly electrically charged to a predetermined potential. This electric charge processing generally uses charging up with corona discharge, in which a corona generated by applying a high voltage to a thin corona discharge wire is made to act on the photosensitive drum surface to charge it.
However, the recent mainstream is a contact charge system which is advantageous in low-voltage process, low ozone generation amount, and low cost. In this contact charge system, for example, a roller charge member (to be referred to as a charge roller hereinafter) is caused to abut against the surface of a photosensitive drum. A voltage is applied to the charge roller to electrically charge the photosensitive drum. To obtain a desired potential Vd on the photosensitive drum surface by the contact charge system, a DC voltage (Vd+Vth) (discharge start voltage (charge start voltage) to the charged body when a DC voltage is applied to the charge member) is applied to the charge roller.
To make the charge further uniform, as disclosed in Japanese Patent Laid-Open No. 63-149668, an “AC charging method” is used. In this method, a voltage (alternating voltage/undulating voltage/oscillating voltage; a voltage whose value changes periodically over time) obtained by superposing an alternating voltage component (AC voltage component) having a peak-to-peak voltage twice or more Vth on a DC voltage corresponding to the desired potential Vd is applied to a contact charge member. In the AC charge method, an AC voltage is applied to alternately cause discharge to the positive and negative sides so that uniform charge can be attained. For example, an AC voltage (oscillating voltage) obtained by superposing an AC voltage which has a peak-to-peak voltage twice or more the discharge start voltage (charge start voltage) of the charged body upon applying a DC voltage on a DC voltage (DC offset bias) is applied. In this case, the charge roller serving as a charged body can be almost uniformly charged, as is known. Assume that an AC voltage having a sine wave is applied to the charge roller. A resistive load current flowing to the resistive load between the charge roller and the photosensitive drum, a capacitive load current flowing to the capacitive load between the charge roller and the photosensitive drum, and a discharge current between the charge roller and the photosensitive drum flow. A current as the sum of these currents flows to the charge roller. To stably charge the charge roller, the discharge current amount preferably has a predetermined value or more, as is empirically known.
FIG. 21 is a graph showing the characteristic of a charge current (Ic) which flows to the charge roller when a charge AC voltage (Vc) is applied to the charge roller. The voltage Vc is indicated by the peak voltage value of the AC voltage. The current Ic is indicated by the effective value of the AC current. The peak voltage value of the AC voltage means a voltage value ½ the peak-to-peak voltage of the AC voltage.
Referring to FIG. 21, when the amplitude (peak voltage value) of the charge AC voltage (Vc) is gradually increased, the charge current (Ic) flows accordingly. When the charge AC voltage (Vc) is equal to or lower than a predetermined voltage (Vth), the amplitude of the charge AC voltage is almost proportional to the charge current. The reason is that the resistive load current and capacitive load current are proportional to the voltage amplitude, and no discharge phenomenon occurs, and no discharge current flows because the voltage amplitude (peak voltage value) is small. When the amplitude of the charge AC voltage (Vc) is further increased, the discharge phenomenon starts at the predetermined voltage (Vth). The charge AC voltage and charge current (Ic) are not proportional any more. The charge current (Ic) becomes larger by an amount corresponding to a discharge current (Is). To obtain stable electric charge, a charge voltage (Vt) is set such that the discharge current (Is) has a predetermined value or more.
However, when the discharge current (Is) to the photosensitive drum increases, photosensitive drum degradation such as wear of the photosensitive drum is accelerated. In addition, abnormal images such as image deletion may occur due to discharge products under a high-temperature high-humidity environment. For this reason, to obtain stable charge and solve the above-described problems, a minimum necessary charge AC voltage is applied to minimize discharge which is alternately caused to the positive and negative sides.
In fact, the relationship between the discharge amount and the applied voltage to the photosensitive drum is not always constant. It changes depending on the thickness of the photosensitive layer or dielectric layer of the photosensitive drum, the charge member, or an environmental variation in air. Under a low-temperature low-humidity environment (to be referred to as an L/L environment hereinafter), discharge hardly occurs because the materials dry, and the resistance values increase. To obtain uniform charge, a peak-to-peak voltage having a predetermined value or more is necessary. Under a high-temperature high-humidity environment (H/H), conversely, even when the minimum charge AC voltage to obtain uniform charge in the L/L environment is applied, discharge more than necessary occurs because the materials absorb moisture, and the resistance values decrease. Since the discharge amount increases, problems such as image formation errors, toner fusion, wear of the photosensitive drum by photosensitive drum surface degradation, and short life are posed.
As is known, such a change in discharge amount occurs not only due to the above-described factors by environmental variations but also due to resistance value variations due to manufacturing variations of charge members or contamination, electrostatic capacity variations of the photosensitive drum caused by endurance, and characteristic variations of the high voltage generator in the image forming apparatus main body.
To suppress the change in discharge amount, the “discharge current control system” disclosed in Japanese Patent Laid-Open No. 63-149668 can change the AC voltage to be applied to the charge member. The AC voltage vs. AC current characteristic is detected in each of a voltage range where the peak voltage of the AC voltage is less than the voltage (Vth) at which the discharge phenomenon starts and a voltage range more than the voltage (Vth). An AC voltage value to obtain an optimum discharge amount is calculated from the two detected characteristic lines. Accordingly, the voltage level of the peak voltage of the AC voltage to be applied to the charge member is determined.
Points A, B, C, and D indicated by circles represent sampling points in FIG. 21. Sampling is executed at the points A and B in the voltage range less than the voltage (Vth) at which the discharge phenomenon starts. Accordingly, a characteristic LINE-A of the charge AC voltage (Vc) and charge current (Ic) in the region where no discharge current is generated is measured. Similarly, sampling is executed at the points C and D after discharge. Accordingly, a characteristic LINE-B of the charge AC voltage (Vc) and charge current (Ic) in the region where the discharge current is generated is measured. A charge AC voltage value to obtain a predetermined discharge current value is calculated on the basis of the relationship between the two characteristic lines obtained by the above-described method. The charge AC voltage is controlled in accordance with the value, thereby suppressing the variation in discharge amount.
FIG. 22 depicts a view showing the timing of the above-described sampling at the sample points in an image forming apparatus using the conventional discharge current control method.
When the main power of the apparatus main body is turned on, a pre-multiple rotation step of a fixing roller is executed to perform a series of processing operations of, e.g., driving the fixing device and heating it to a predetermined temperature. Then, a standby state is set. When a print start instruction is received from an external device such as an external personal computer, a pre-rotation step as predetermined print preparation is executed. After that, a print step of executing the print operation on a printing paper sheet by a series of electrophotographic processes starts. In a mode for executing the print operation on a plurality of printing paper sheets, predetermined processing is executed in a paper feed interval step until the print operation for the next printing paper sheet. Then, the print step for the second and subsequent paper sheets starts. When the print step for the last printing paper sheet is ended, a post-rotation step is executed, and the standby state is set again. The above-described sampling at the points A and B, and C and D is executed in the pre-rotation step. After the charge AC voltage level set on the basis of the sampling result, the print step starts. When sampling is executed at timings except the print steps, any errors such as abnormal images generated by the charge voltage equal to or lower than the discharge start voltage (Vth) during sampling can be prevented.
In the conventional discharge current control method, however, when a “continuous print mode” is set to continuously execute print processing for a plurality of printing paper sheets, the value of the discharge current varies during the print operation. This problem is posed when the temperature near the photosensitive drum rises during the print operation in the continuous print mode, and the relationship between the discharge current and the applied voltage to the charge roller changes. More specifically, the characteristic lines LINE-A and LINE-B shown in FIG. 21 vary during the print operation. Even when the charge AC voltage which is set in accordance with the sampling result at the start of printing is applied, the discharge current cannot be controlled to the desired value. To solve this problem, the print operation is stopped at a predetermined interval for a predetermined period in the continuous print mode to decrease the peak voltage of the charge AC voltage to the discharge start voltage (Vth) or less. Then, sampling is executed again, and the charge voltage output to obtain the optimum discharge current is set again. However, this measure is not effective because the print speed of the image forming apparatus decreases.
In the image forming apparatus using the conventional discharge current control method, since the scale of the high voltage circuit which generates the charge AC voltage is large, the following problems rise.
(1) The manufacturing cost of the high voltage circuit is high.
(2) The high voltage circuit is bulky, and consequently, the image forming apparatus becomes bulky.
A charge bias circuit using the conventional discharge current control method superposes a DC bias on the charge AC voltage and outputs it from the output terminal. The output terminal is connected to the charge roller. In the conventional discharge current control method, the discharge current is calculated by detecting the relationship between the AC output voltage and the AC output current. Hence, the charge bias circuit includes a detection circuit which detects the level of the AC output voltage. The AC output voltage detection circuit includes a number of electric components, and many of them generate large potential differences between the terminals. In addition, large potential differences are generated between the terminals of capacitors, diodes, and resistors used in this circuit. For these reason, components of high dielectric breakdown voltage specifications which can stand a large potential difference must be used.
Since such electric components of high dielectric breakdown voltage specifications are generally expensive, the manufacturing cost of the circuit becomes high. In a component which generates a large potential difference, the distance between the terminals must be large to ensure the insulating properties between them. In addition, even the distance between the components must also be large to ensure the insulating properties between them. As a result, the circuit scale becomes large.